using System;
using System.Collections.Generic;
using System.Security.Cryptography;
public sealed class Crc32 : HashAlgorithm
{
    public const UInt32 DefaultPolynomial = 0xedb88320u;
    public const UInt32 DefaultSeed = 0xffffffffu;

    static UInt32[] defaultTable;

    readonly UInt32 seed;
    readonly UInt32[] table;
    UInt32 hash;

    public Crc32() : this(DefaultPolynomial, DefaultSeed) { }

    public Crc32(UInt32 polynomial, UInt32 seed)
    {
        if (!BitConverter.IsLittleEndian)
            throw new PlatformNotSupportedException("Not supported on Big Endian processors");

        table = InitializeTable(polynomial);
        this.seed = hash = seed;
    }

    public override void Initialize()
    {
        hash = seed;
    }

    protected override void HashCore(byte[] array, int ibStart, int cbSize)
    {
        hash = CalculateHash(table, hash, array, ibStart, cbSize);
    }

    protected override byte[] HashFinal()
    {
        var hashBuffer = UInt32ToBigEndianBytes(~hash);
        HashValue = hashBuffer;
        return hashBuffer;
    }

    public override int HashSize { get { return 32; } }

    public static UInt32 Compute(byte[] buffer)
    {
        return Compute(DefaultSeed, buffer);
    }

    public static UInt32 Compute(UInt32 seed, byte[] buffer)
    {
        return Compute(DefaultPolynomial, seed, buffer);
    }

    public static UInt32 Compute(UInt32 polynomial, UInt32 seed, byte[] buffer)
    {
        return ~CalculateHash(InitializeTable(polynomial), seed, buffer, 0, buffer.Length);
    }

    static UInt32[] InitializeTable(UInt32 polynomial)
    {
        if (polynomial == DefaultPolynomial && defaultTable != null)
            return defaultTable;

        var createTable = new UInt32[256];
        for (var i = 0; i < 256; i++)
        {
            var entry = (UInt32) i;
            for (var j = 0; j < 8; j++)
                if ((entry & 1) == 1)
                    entry = (entry >> 1) ^ polynomial;
                else
                    entry = entry >> 1;
            createTable[i] = entry;
        }

        if (polynomial == DefaultPolynomial)
            defaultTable = createTable;

        return createTable;
    }

    static UInt32 CalculateHash(UInt32[] table, UInt32 seed, IList<byte> buffer, int start, int size)
    {
        var hash = seed;
        for (var i = start; i < start + size; i++)
            hash = (hash >> 8) ^ table[buffer[i] ^ hash & 0xff];
        return hash;
    }

    static byte[] UInt32ToBigEndianBytes(UInt32 uint32)
    {
        var result = BitConverter.GetBytes(uint32);

        if (BitConverter.IsLittleEndian)
            Array.Reverse(result);

        return result;
    }
}